1. Field of the Invention
The present invention relates to a zoom lens system, particularly, it relates to a zoom lens system applicable in small-sized cameras such as electronic still cameras, video cameras and so on.
2. Description of the Prior Art
In recent years, an image pick-up device, electric circuit etc. have become compact considerably and less expensive, since packaging and integration rate thereof have improved. As a result, a volume, weight and cost of a lens system occupying in an electronic still camera, video camera and the like are increasing relatively, thus it has been a strong demand to make the lens compact and to reduce the cost.
Further, a glass plate such as a low-pass filter, half prism or the like is provided at an image side next to lens elements in the electronic still camera or video camera, so that a very long back focal length is required compared with that of the lens system used in single reflex cameras.
In addition, recently, as the resolution of an image pick-up device such as CCD is improved, a high performance must be satisfied throughout a whole image plane.
In a zoom lens system, though there are various conventional types such as a 2-components, 3-components, 4-components or more zoom lens system, it is practically inevitable that the configuration becomes complicated, cost increases and the size becomes larger due to the increase of the number of lens elements as the component increases. Therefore, in order to attain the reduction of the number of lens elements, simplification of the configuration, cost reduction and smaller size, the 2-components zoom lens system is the most effective. However, a zoom lens system used frequently in a lens shutter camera and the like, and having, from object side, a positive lens unit and a negative lens unit is not applicable because of a very short back focal length. On the contrary, as 2-components zoom lens system having, from object side, negative lens unit and a positive lens unit, the ones disclosed in Japanese Patent Laid-Open No. 60246/1978, Japanese Patent LaidOpen No. 114236/1979 and Japanese Patent Publication No. 46809/1986 are known. Though these zoom lens systems are configured with comparatively small numbers of lens elements, that is, six or seven lens elements, they are hard to be said that the zone ratios and brightness are satisfactory and that the back focal lengths are sufficiently long.
Next, considering the whole lens system including an aperture diaphragm, in a zoom lens system having from object side, a negative lens unit and a positive lens unit, the aperture diaphragm is generally arranged in or before or behind the rear lens unit and shiftable along the optical axis of the lens system in a zooming operation. In the electronic still camera, video camera and the like, since the aperture diaphragm is controlled electrically, the space required for the arrangement of the aperture diaphragm becomes larger and further the weight becomes heavier due to a driving system such as a motor and so on. Therefore, in order to make such aperture diaphragm shiftable along the optical axis of the lens system in the zooming operation, a large space must be reserved in a lens barrel for shifting of the aperture diaphragm, thereby compactness of the whole system including the lens barrel becomes difficult to be attained. Furthermore, since a large weight component must be shifted in the zooming operation, a powerful motor is required, which undesirably results in a high cost and electric power consumption.
On the contrary, for example, a 2-components zoom lens system having a stationary aperture diaphragm behind a rear lens unit is disclosed in Japanese Patent Laid-Open No. 35334/1980. In this type, though an entrance pupil is brought nearest to object side to prevent the diameter of a front lens unit from becoming large, since the refractive power balance between the front and rear lens units is not proper, reduction of peripheral illuminance at the shortest focal length condition is large, therefore, in order to prevent this, the diameter of the front lens unit becomes large compared with its zoom ratio. In addition, this system is hard to be said that compactness has been satisfactorily attained because of its configuration with such a comparatively large number as 8 of lens elements.